As one type of conventional vehicle lighting units such as a vehicle headlamp, a lighting unit utilizing a semiconductor light emitting element as a light source together with a wavelength conversion material such as a phosphor has been known (see, for example, Japanese Patent No. 4124445). With this type of vehicle lighting unit, the semiconductor light emitting element can emit light such as blue light, so that the phosphor can be irradiated with the blue light. Therefore, the phosphor can be excited to emit light such as yellow light. The blue light originated from the semiconductor light emitting element and the yellow light from the phosphor can be mixed to produce visible light such as white light. The visible light can be illuminated forward the vehicle body by means of an optical system including a reflector and the like.
In order for such a vehicle lighting unit to provide higher luminance irradiation light, a semiconductor laser light source that can emit higher luminance laser light may be utilized as the light source semiconductor light emitting element.
However, in the above conventional vehicle lighting unit, when excitation light is made incident on the phosphor from the light extraction direction of the phosphor, part of the excitation light can be reflected off the surface of the phosphor. That part of light may exit from the vehicle lighting unit without color mixture, thereby generating color unevenness in the light distribution pattern formed by the vehicle lighting unit. (That is the projection image by the vehicle lighting unit.)
When a semiconductor laser light source is used as the semiconductor light emitting element, almost all or substantially all the laser light (excitation light) emitted from the light source can be scattered by the phosphor to lose its coherency. Part of the laser light, however, can be reflected off the surface of the phosphor as described above and exit from the vehicle lighting unit with its coherency maintained. Therefore, if the power density thereof is made larger than the maximum permission exposure, deterioration of the usefulness of the semiconductor laser light source as a light source may occur.